Method and apparatus for generating a synchronization sequence in a spread spectrum communications transceiver

ABSTRACT

A novel and useful acquisition and synchronization mechanism for spread spectrum communication systems whereby a synchronization sequence comprising a plurality of known symbols spaced apart by predefined time delay intervals is transmitted as the start of packet signal. At the transmitter, a synchronization sequence is transmitted at the beginning of each packet. A synchronization sequence is generated which includes a plurality of symbols with predefined time gaps between each of the symbols. Multiple synchronization sequences may be generated wherein each sequence comprises a unique set of time delays or gaps between each of the symbols. Each set of unique time delays or gaps between symbols of a sequence is stored as a synchronization sequence gap template in memory. When required to generate a synchronization sequence, the sequence generator outputs the plurality of synchronization symbols and inserts a specific time delay between each of the symbols in accordance with the contents of the gap template for the particular synchronization sequence.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.09/933,065, filed Aug. 20, 2001, entitled “Acquisition ofSynchronization in a Spread Spectrum Communications Transceiver,” nowU.S. Pat. No. 6,727,790.

FIELD OF THE INVENTION

The present invention relates generally to data communication systemsand more particularly relates to a method and apparatus for generating asynchronization sequence in a spread spectrum communicationstransceiver.

BACKGROUND OF THE INVENTION

The use of spread spectrum communications techniques to improve thereliability and security of communications is well known and is becomingincreasingly common. Spread spectrum communications transmits datautilizing a spectrum bandwidth that is much greater than the bandwidthof the data to be transmitted. This provides for a more reliablecommunication in the presence of high narrowband noise, spectraldistortion and pulse noise, in addition to other advantages. Spreadspectrum communication systems typically utilize correlation techniquesto identify an incoming received signal.

Spread spectrum communications systems were used for use in militaryenvironments to overcome high-energy narrowband enemy jamming. Incommercial or home environments, they may be used to achieve reliablecommunication on noise media such as the AC power line. In particular,certain home electrical appliances and devices can potentially be verydisruptive of communications signals placed onto the power line. Forexample, electronic dimming devices can place large amounts of noiseonto the power line since these devices typically employ triacs orsilicon controlled rectifiers (SCRs) to control the AC waveform inimplementing the dimming function.

A communication medium such as the AC power line may be corrupted byfast fading, unpredictable amplitude and phase distortion and additivenoise. In addition, communication channels may be subjected tounpredictable time varying jamming and narrowband interference. In orderto transmit digital data over such channels it is preferable to use aswide a bandwidth as possible for transmission of the data. This can beachieved using spread spectrum techniques.

The spread spectrum receiver is required to perform synchronization thatis commonly achieved using some form of acquisition method optionally incombination with a tracking loop or other tracking mechanism. In a noisyunpredictable environment such as the AC power line, the tracking looptypically fails frequently causing loss of information. Communicationsystems to overcome these problems are large, complex and expensive.

Synchronization of signals between the transmitter and the receiver thatare communicating with each other in a spread spectrum communicationsystem is an important aspect of the process of transmitting signalsbetween them. Synchronization between transmitter and receiver isnecessary to allow the despreading of the received signals using aspreading code that is synchronized between them so that the originallytransmitted signal can be recovered from the received signal.Synchronization is achieved when the received signal is accurately timedin both its spreading code pattern position and its rate of chipgeneration with respect to the receiver's spreading code.

One of the problems associated with synchronization is that thetechniques used to synchronize two signals are relatively expensive toimplement. In communication systems having sophisticated and relativelyexpensive central communication sites which serve a plurality ofrelatively inexpensive remote communication sites, it is desirable toreduce the cost of synchronization systems in the remote communicationsites while not increasing the cost of the central communication sites.

In a communications transceiver, it is desirable that the acquisitionmechanism be more reliable that any error correction code used for thedata portion of the packet. In other words, it is preferable to declaresynchronization correctly and not be able to correctly decode the packetdata than to miss the entire packet altogether because of a weakacquisition algorithm.

Further, it is desirable that the acquisition algorithm has as low aprobability as possible of false synchronization from noise, e.g., lessthan once in 5 seconds. The acquisition mechanism should be capable ofutilizing more than one synchronization sequence whereby the probabilityof synchronization from another sequence is minimized.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a novel and useful mechanismfor generating a synchronization sequence in a spread spectrumcommunications transmitter. The mechanism of the present invention isuseful in communication systems characterized by shared media such asnetworks that use power line carrier communications. In general, theinvention is applicable where a plurality of stations are connected to ashared communication media whereby receiving stations must acquiresynchronization on a start of packet signal transmitted by transmittingstations at the beginning of each packet.

An improved acquisition mechanism for spread spectrum communicationsystems is provided whereby a synchronization sequence comprising aplurality of known symbols spaced apart by predefined time delayintervals is transmitted as the start of packet signal. At thetransmitter, a synchronization sequence is transmitted at the beginningof each packet. A synchronization sequence is generated which includes aplurality of symbols with predefined time gaps between each of thesymbols. Multiple synchronization sequences may be generated whereineach sequence comprises a unique set of time delays or gaps between eachof the symbols. Each set of unique time delays or gaps between symbolsof a sequence is stored as a synchronization sequence gap template inmemory. When required to generate a synchronization sequence, thesequence generator outputs the plurality of synchronization symbols andinserts a specific time delay between each of the symbols in accordancewith the contents of the gap template for the particular synchronizationsequence.

At the receiver, the received signal is correlated against thesynchronization sequence using the predefined gaps or time delayintervals inserted between the symbols. The received signal is firstpassed through a linear correlator which functions to generate acorrelation peak for each symbol received. The expected position of eachcorrelation peak is then calculated and compared to the positions of thecorrelation peaks received. If the number of matches exceeds athreshold, synchronization is declared.

The acquisition algorithm is adapted to search for matching correlationpeaks while considering zero or more received symbols in error. Further,the algorithm permits a match if the expected correlation position iswithin a predefined delta of the received correlation peak. Ifunsuccessful, the acquisition algorithm repeats in an attempt tocorrelate each predefined synchronization sequence to the receivedcorrelation peaks.

Once synchronization is declared, a synchronization quality factor iscalculated as a function of the number of matches and the number ofcorrelation peaks whose value exceeds a threshold. If subsequentsynchronizations are declared, the quality factors are compared and ifthe latest quality factor is greater, the previous packet is dropped andthe current packet is received. Note that the process of comparingsynchronization quality and dropping the previous packet in favor of thenext packet is performed until the header CRC checksum field isverified. After the header CRC is checked and verified, the receiver islocked into receiving the current packet.

The acquisition mechanism provides for multiple synchronizationsequences wherein the cross correlation of the sequences is minimized inorder to reduce the probability of false detection with anothersequence. The use of multiple synchronization sequences permitsadditional information to be transmitted to the receiving station. Forexample, the different sequences may be adapted to indicate to thereceiver the particular packet type or modulation scheme used for thatpacket transmission.

Many aspects of the previously described invention may be constructed assoftware objects that execute in embedded devices as firmware, softwareobjects that execute as part of a software application on a computersystem running an operating system such as Windows, UNIX, LINUX, etc.,an Application Specific Integrated Circuit (ASIC) or functionallyequivalent discrete hardware components.

There is thus provided in accordance with the present invention, amethod of generating a start of packet synchronization sequence for usein a transmitter, the method comprising the steps of generating aplurality of N symbols to be transmitted in the synchronizationsequence, generating N-1 predetermined signals, inserting one of the N-1predetermined signals after each of the first N-1 symbols in thesynchronization sequence and wherein N is a positive integer.

There is also provided in accordance with the present invention, amethod of generating a start of packet synchronization sequence for usein a code shift keying (CSK) based transmitter, the method comprisingthe steps of generating a plurality of symbols of known shift rotationto be transmitted in the synchronization sequence, inserting apredetermined time delay between each of the symbols and wherein thepredetermined time delays inserted between the symbols define a uniquesynchronization sequence gap template.

There is further provided in accordance with the present invention, atransmitter for use in a spread spectrum communications systemcomprising synchronization sequence generator adapted to generate asynchronization sequence, the synchronization sequence representing aplurality of synchronization symbols with predetermined time delaysinserted therebetween, an encoder adapted to determine a shift index tobe applied to a spreading waveform, the shift index determined based onthe synchronization sequence, a spreading waveform generator adapted togenerate a spreading waveform signal in accordance with the shift indexand wherein delays between spreading waveform signals are determined bythe predetermined time delays in the synchronization sequence.

There is also provided in accordance with the present invention, acommunications station for transmitting and receiving signals to andfrom other stations connected over a shared communications media basednetwork comprising a coupling circuit for generating a receive signalreceived over the network and for outputting a transmit signal onto thenetwork, a transmitter adapted to modulate a synchronization sequenceand data to be transmitted in accordance with a modulation scheme so asto generate the transmit signal therefrom, wherein the transmittercomprises means for generating a plurality of symbols of known shiftrotation to be transmitted in the synchronization sequence and means forinserting a predetermined time delay between each of the symbols, areceiver adapted to demodulate the receive signal in accordance with themodulation scheme so as to generate a receive data signal therefrom, amedia access control (MAC) circuit adapted to interface an applicationprocessor to the shared communications media and the applicationprocessor adapted to control the operation of the transmitter, receiverand MAC and to provide an interface between the MAC and an externalhost.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is a diagram illustrating an example transmitter adapted togenerate a synchronization sequence constructed in accordance with thepresent invention;

FIG. 2 is a diagram illustrating an example receiver comprising anacquisition and synchronization circuit constructed in accordance withthe present invention;

FIG. 3 is a diagram illustrating the format of an example packetcomprising a synchronization sequence;

FIG. 4 is a diagram illustrating an example synchronization sequencetransmission signal comprising a plurality of symbols separated bypredetermined time delays;

FIG. 5 is a diagram illustrating the corresponding correlation peaksgenerated in response to the synchronization sequence of FIG. 4;

FIG. 6 is a diagram illustrating the output of the linear correlator inresponse to an example received signal corresponding to thesynchronization sequence transmission signal of FIG. 4;

FIGS. 7A and 7B are a flow diagram illustrating the acquisition methodof the present invention in more detail;

FIG. 8 is a block diagram illustrating an example embodiment of astation incorporating transmitter and receiver circuits adapted toperform the acquisition and synchronization mechanisms of the presentinvention; and

FIG. 9 is a flow diagram illustrating the transmit method of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION Notation Used Throughout

The following notation is used throughout this document.

Term Definition AC Alternating Current ASIC Application SpecificIntegrated Circuit BPF Band Pass Filter CD Carrier Detect CRC CyclicRedundancy Code CSK Code Shift Keying CSMA Carrier Sense Multiple AccessDCSK Differential Code Shift Keying DSP Digital Signal Processor EEROMElectrically Erasable Read Only Memory FPGA Field Programmable GateArray IR Infrared ISO International Standards Organization MAC MediaAccess Control OSI Open Systems Interconnect PBX Private Branch ExchangePLC Power Line Carrier RAM Random Access Memory RF Radio Frequency ROMRead Only Memory SCR Silicon Controlled Rectifier UST Unit Symbol Time

Detailed Description of the Invention

The present invention is a novel and useful mechanism for generating asynchronization sequence in a spread spectrum communicationstransmitter. The mechanism of the present invention is useful incommunication systems characterized by shared media such as networksthat use power line carrier communications. In general, the invention isapplicable where a plurality of stations are connected to a sharedcommunication media whereby receiving stations must acquiresynchronization on a start of packet signal transmitted by transmittingstations at the beginning of each packet.

An improved acquisition mechanism for spread spectrum communicationsystems is provided whereby a synchronization sequence is generated atthe transmitter. The synchronization sequence includes a plurality ofsymbols with predefined time gaps between each of the symbols and istransmitted at the beginning of each packet as the start of packetsignal. Multiple synchronization sequences may be generated wherein eachsequence comprises a unique set of time delays or gaps between each ofthe symbols. Each set of unique time delays or gaps between symbols of asequence is stored as a synchronization sequence gap template in memory.When required to generate a synchronization sequence, the sequencegenerator outputs the plurality of synchronization symbols and inserts aspecific time delay between each of the symbols in accordance with thecontents of the gap template for the particular synchronizationsequence.

At the receiver, the received signal is correlated against thesynchronization sequence using the predefined gaps or time delayintervals inserted between the symbols. The received signal is firstpassed through a linear correlator which functions to generate acorrelation peak for each symbol received. The expected position of eachcorrelation peak is then calculated and compared to the positions of thecorrelation peaks received. If the number of matches exceeds athreshold, synchronization is declared.

For purposes of this specification, the term ‘station,’ ‘node’ or‘communication node’ shall be taken to mean any network entity,implemented in either hardware, software or a combination of hardwareand software, which may be the endpoint of a call, link or connectionwithin a shared media based network. The network may comprise any typeof shared network or media including but not limited to power linecarrier based networks, twisted pair networks, IR wireless networks, RFwireless networks, optical fiber ring networks, etc. The term ‘call,’‘link’ or ‘connection’ shall be taken to mean any communication paththat is established between at least two nodes for the purpose ofcommunication therebetween. The term phase unit is defined as a sampletime in the receiver. A sample time is any suitable period that thesignal or correlator output can be sampled without loosing information.

The synchronization sequence generation mechanism of the presentinvention is especially suited for use in a spread spectrum datacommunications system that utilizes the Differential Code Shift Keying(DCSK) or non-differential Code Shift Keying (CSK) modulation technique.Such communications systems are applicable to relatively noisyenvironments such as the AC power line.

In a CSK transmission system, the data is transmitted in the form oftime shifts between consecutive circularly rotated waveforms of length Twhich are referred to as spreading waveforms, i.e., spread spectrumcorrelator sequence waveforms. The spreading waveforms can comprise anytype of waveform that has suitable auto correlation properties. Duringeach symbol period, referred to as a unit symbol time (UST), a pluralityof bits are transmitted. The symbol period is divided into a pluralityof shift indexes with each shift index representing a particular bitpattern. The information, i.e., bit pattern, is conveyed by rotating thespreading waveform by a certain amount corresponding to the data to betransmitted. The data is conveyed in the degree of rotation or circularshift applied to the spreading waveform before it is transmitted. Notethat the spreading waveform may comprise any suitable waveform such as achirp, pseudorandom sequence, etc.

In a CSK system, the data is conveyed in the absolute shift assigned tothe spreading waveform. In a DCSK system, the data is conveyed in theshift differential between consecutive symbols. The synchronizationscheme of the present invention is applicable to both CSK and DCSKtransmission systems.

Upon reception by the receiver, the signal is input to a matched filterhaving a template of the spreading waveform pattern to detect the amountof rotation (or circular shift) within the received signal for eachsymbol. The received data is fed into a cyclic correlator wherein thecontents are periodically circularly shifted and a correlation outputgenerated therefrom. Cyclic correlation may be achieved by inputting thereceived data to a shift register whose output is fed back to its inputand circularly rotating, i.e., shifting, the contents of the shiftregister. The output of the shift register is input to a matched filter.For each bit shift or rotation, the matched filter generates acorrelation sum. A shift index is determined for each UST correspondingto the shift index that yields the maximum (or minimum) correlation sum.Differential shift indexes are generated by subtracting the currentlyreceived shift index from the previously received shift index. Thedifferential shift index is then decoded to yield the originallytransmitted data.

Spread spectrum communications systems based on DCSK or CSK modulationare described in more detail in U.S. Pat. No. 6,064,695, to Raphaeli,entitled “Spread Spectrum Communication System Utilizing DifferentialCode Shift Keying,” incorporated herein by reference in its entirety.

Transmitter with Synchronization Sequence Generator

Transmitting stations transmit data in the form of packets to receivingstations. Each packet is preceded by a synchronization sequencecomprising a predetermined number of symbols having predefined gaps ortime delays between each symbol. The length of the synchronizationsequence can be any suitable number of symbols such that receivingstations are able to synchronize with the transmitting station. Forillustration purposes only, in the example presented herein, thesynchronization sequence comprises a sequence of seven symbols of knownshift rotation, e.g., zero shift symbols. The seven symbols aretransmitted whereby a specific predetermined time delay is insertedbetween each of the symbols. The particular time delays inserted betweenthe symbols define a unique synchronization sequence gap (i.e. timedelay) template. Different synchronization sequences have different timedelay templates. The intervals inserted between synchronization symbolsare used by the receiver in the receiving station to determine thespecific packet type used in the transmission. Knowledge of the type ofpacket is crucial to be able to correctly decode the remainder of thepacket.

A diagram illustrating an example transmitter adapted to generate asynchronization sequence constructed in accordance with the presentinvention is shown in FIG. 1. The transmitter, generally referenced 10,is typically part of a modem transceiver located in each station. In theexample provided, the modem transceiver is adapted to communicate usingthe CSK modulation scheme described hereinabove. Note that one skilledin the communication arts may apply the techniques of the presentinvention to other modulation techniques as well.

Data to be transmitted is provided by an external host 12 and input toan encoder 14. The encoder functions to determine the amount of rotationto be applied to the output spreading waveform. The amount of rotationis represented as a shift index. The shift index is input to thespreading waveform generator 20 which functions to generate thespreading waveform signal in accordance with the shift index. Thespreading waveform itself is stored in a spreading waveform ROM 24 whichcontains the digitized representation of the spreading waveformfrequency waveform. The spreading waveform is read out starting from aninitial point corresponding to the shift index. Starting from theinitial point, the entire spreading waveform is circularly read out andtransmitted onto the channel via the coupling circuitry 22.

The coupling circuitry comprises the circuitry required to couple thesignal onto the physical channel. For example, the coupling circuitrycomprises a D/A converter whose analog output is first filtered by aband pass filter (BPF) having a suitable pass band in accordance withthe signal width. The output of the BPF is then amplified by an outputamplifier wherein the output of the amplifier comprises the transmitoutput signal.

The transmitter functions not only to transmit data but also thesynchronization sequence which forms the start of packet signal that istransmitted at the beginning of each packet. The synchronizationsequence is generated by the synchronization sequence generator 16 whoseoutput is input to the encoder along with the data received from thehost. The encoder is adapted to process either the data from the host orsynchronization sequence from the synchronization sequence generator inaccordance with a sync/data control signal output by a controller 26.

When in synchronization mode, the encoder is operative to generate shiftindexes in accordance with the input synchronization sequence. Inaccordance with the invention, the synchronization sequence comprises aplurality of symbols with predefined time gaps between each of thesymbols. Multiple synchronization sequences may be generated whereineach sequence comprises a unique set of time delays or gaps between eachof the symbols. One of a plurality of synchronization sequences may beselected using the SEQ control signal output from the controller.

Each set of unique time delays or gaps between symbols of a sequence isstored as a sync sequence gap template in a ROM or other table means 18.When required to generate a synchronization sequence, the sequencegenerator outputs the plurality of synchronization symbols (e.g.,symbols with zero or other rotation to both transmitter and receiver)and inserts a specific time delay between each of the symbols inaccordance with the contents of the gap template for the particularsynchronization sequence to be transmitted. Note that for the examplecase of seven symbols per synchronization sequence, the sync sequencegap table is adapted to store six time delays per synchronizationsequence. The time delays may be stored in any suitable format, e.g.,units of time, clocks, fractions of a UST, phase clock ticks, etc.

A diagram illustrating an example synchronization sequence transmissionsignal comprising a plurality of symbols separated by predetermined timedelays is shown in FIG. 4. Each symbol has a fixed length of one USTwhich in this example is equivalent to 256 receiver correlator phaseunits (i.e. sample times) which corresponds to 800 μs. The time delayinserted between each symbol has a maximum length of 700 μs. In theexample shown, the first delay inserted after the first symbol is 64phase units or 200 μs.

In accordance with the invention, a set of five orthogonalsynchronization sequences are provided wherein each sequence is used toconvey information about the packet type used in the particulartransmission. Each packet type corresponds to a differentsynchronization sequence. The five synchronization sequences are listedbelow in Table 1.

TABLE 1 Synchronization Sequence Time Delay Intervals Sequence PacketNumber Type Time Delays 1 1 [10, 6, 12, 9, 8, 5] 2 2 [4, 5, 14, 12, 11,9] 3 3 [6, 4, 11, 13, 5, 10] 4 4 [8, 7, 4, 9, 5, 11] 5 5 [11, 5, 7, 9,6, 6]

The packet types may correspond, for example, to packets of differentdata rates, ACK packets, etc. The different synchronization sequencesmay be used to convey any type of information depending on theimplementation and is not limited to conveying the modulation or packettype. The time delays for each sequence are presented as multiples of 50μs. Thus, the delay inserted between the third fourth symbol forsequence #3 is 550 is. The total duration of the synchronizationsequence is equal to the sum of the time delays and the sum of the sevensymbol durations. Note that the sum of the time delays for any of thesequences above does not exceed 64 (i.e. 4 USTs or 3,200 μs). Thus, themaximum duration for the synchronization sequence is 11 USTs comprisedof the seven symbol USTs plus the four USTs of intersymbol delays.

Note that the gap may be zero and may be larger than 14. In addition,the mechanism may be implemented using any type of signal and is notlimited to the use of gaps. Further, variable length symbols may be usedwhereby correlation is performed on only a portion of the receivedsymbol. In other words, energy may be added before or after thetransmitted symbol wherein the correlation length remains fixed.Alternatively, the symbols may be rotated rather than having zero shift.In this case, the rotation causes the correlation point of the symbol tomove and causes the correlation value to be reduced in proportion to theamount of rotation applied.

Since the symbol length is 800 μs or 256 phase units (i.e. correlationsamples), the synchronization sequence time delays may be rewritten interms of correlation phase units as presented below in Table 2. Eachsymbol duration is comprised of 256+(time delay * 16) phase units wherethe time delay is from Table 1 above.

TABLE 2 Synchronization Sequence Time Delay Intervals Sequence NumberSymbol Durations 1 [416, 352, 448, 400, 384, 336] 2 [320, 336, 480, 448,432, 400] 3 [352, 320, 432, 464, 336, 416] 4 [384, 368, 320, 400, 336,432] 5 [432, 336, 368, 400, 352, 352]Note that the sync symbol gap ROM in the transmitter may be adapted tostore the time delays, symbol durations or any other value that yieldsthe duration of each symbol and the gap to be inserted between each ofthe symbols. A set of time delays, symbol durations, etc. is providedfor each unique synchronization sequence to be transmitted.

As described above, the synchronization sequence is transmitted at thestart of each packet before the packet data is sent. A diagramillustrating the format of an example packet comprising asynchronization sequence is shown in FIG. 3. The packet, generallyreferenced 70, comprises the synchronization sequence 72 at the start ofthe packet, a packet header field 74, CRC8 error checking value 76, datapayload 78 and CRC16 error checking field 80. As described in moredetail infra, acquisition circuits in the receiver function to acquiresynchronization on the synchronization sequence. Once synchronization isachieved, data decoding can proceed beginning with the packet header.

Once the sync sequence and corresponding shift indexes are determined,the spreading waveform is generated in accordance with the shift index(step 186). The packet, including sync sequence and data payload, isassembled for transmission (step 188) and the signal is coupled to thechannel (step 190).

Receiver with Acquisition and Synchronization Circuit

A diagram illustrating an example receiver comprising an acquisition andsynchronization circuit constructed in accordance with the presentinvention is shown in FIG. 2. The receiver, generally referenced 30,performs both data decoding and acquisition of synchronization on thestart of packet synchronization sequence transmitted before each packet.The signal received from the channel media is input to a channelcoupling circuit 32 which interfaces the receiver to the powerline, etc.The received signal is then filtered by a band pass filter (BPF) 33having suitable frequency characteristics for the band of interest. Thebandwidth of the band pass filter (BPF) is wide enough to receive therange of frequencies transmitted within the spreading waveform. Theoutput of the filter is input to a one-bit A/D converter 34. The A/Dconverter may comprise a comparator in combination with a samplerclocked at a suitable sampling frequency.

The output of the A/D converter is input to one of two inputs ofmultiplexer (mux) 36. The output of the multiplexer is input to a shiftregister 38. For illustrative purposes only, the length of the shiftregister is 256 bits long each. The output of the shift register isinput to a correlator 40. The correlator is implemented using a matchedfilter which functions to recognize the spreading waveform pattern. Thespreading waveform pattern is stored as a template within the correlatorand is used to detect the presence of spreading waveforms from thereceived signal. The serial output of the shift register wraps around tothe second input of the multiplexer. The multiplexer select output iscontrolled by a linear/cyclic control signal output by the controller54.

The correlator circuit is capable of operating in either a linear orcyclic mode. For acquisition and synchronization, the correlator is setto operate in a linear mode of operation. In linear mode operation, themultiplexer is set to select the output of the AID converter as theinput to the shift register. Each bit output of the A/D converter isclocked into the shift register and the parallel output of the shiftregister is input to the correlator. Within the correlator, each bitinput to the correlator is multiplied by a corresponding bit from thetemplate. All 256 products are summed to form the output 62 of thecorrelator.

The output of the linear correlator is input to a correlation peakprocessor 58 functioning as a I²+Q² type energy detector. The functionof the correlation peak processor comprises performing a search over aUST period for the maximum correlation peak and generating a signal 66therefrom, generating the symbol clock 68 and detecting the presence ofcarrier and generating a carrier detect (CD) signal 64 therefrom. The CDsignal is derived from the results of the correlation and is declared ifthe correlation results exceed a threshold.

The output signal 66 of the peak processor is input to the acquisitionprocessor circuit 50 in the acquisition and sync circuit 42. Theacquisition process functions to receive and store the correlation peakdata in a memory table 44 which may comprise any suitable memory means,e.g., RAM, etc. Similarly, the positions of the correlation peaks arealso stored in a memory table 46. A sync sequence template memory (e.g.,ROM, etc.) 48 stores one or more sets of time delays or intersymbolgaps, wherein each set corresponds to a different synchronizationsequence. The acquisition process is described in more detail infra.

Once synchronization is achieved, the controller switches the mode ofoperation of the correlator to cyclic correlation wherein the contentsof the shift register are loaded and circularly shifted for a full USTcycle. The shift yielding the maximum correlation peak is decoded bydata decode circuit 52 and the receive data output therefrom. Thereceiver has knowledge of the location of the symbols (i.e. USTs) in thereceive signal from the output of the synchronization signal 60 outputof the acquisition processor and used by the controller to provide theappropriate timing time the circular correlation process.

Acquisition and Synchronization Mechanism

In accordance with the mechanism of the present invention, correlationof the received signal is performed with the entire synchronizationsequence. Correlation of the received signal with the synchronizationsequence, however, is applied to the signal output from the linearcorrelator 40 by the acquisition processor 50. Processing a receivedsignal in the form of the synchronization sequence transmission signalshown in FIG. 4, yields an output signal from the linear correlatorcomprising a plurality of peaks whereby the distance between the peakscorrespond to the time delays injected between each of the symbols ofthe synchronization sequence. This distances between the peaks are equalto 256 phase units plus the delay as defined by the particularsynchronization sequence template used to generate the transmission.Thus, the main function of the acquisition processor is to search forcorrelation peaks in their respective correct positions, determine thenumber of matching peaks and decide whether to declare synchronizationor not. In other words, the acquisition processor decides whether or nota transmission is currently being received, and if so to proceed todecode the remainder of the packet.

A diagram illustrating the corresponding correlation peaks generated inresponse to the synchronization sequence of FIG. 4 is shown in FIG. 5.The synchronization sequence shown corresponds to Sequence Number 2 inTables 1 and 2 above. Passing the transmission signal of Sequence Number2 through the linear correlator results in a sequence of correlationpeaks spaced apart by distances in accordance with Sequence number 2.Thus, the six gaps or symbol interval time delays correspond to those inTable 2 for Sequence Number 2.

The synchronization point whereupon data decoding begins is taken 500 μs(i.e. 160 phase units) after the last peak as shown by the verticalarrow. The switching point from linear to cyclic correlation occurs abit later 1300 μs after the last correlation peak. The 1300 μs time isdefined from the time of reception of the last symbol of thesynchronization sequence (i.e. location of the correlation peak) plus a500 μs delay following the peak followed by the first symbol of the data(i.e. 800 μs). Cyclic correlation begins only after the first datasymbol has been clocked into the shift register.

Each unique synchronization sequence determines where the sevencorrelation peaks are expected to be located. The six predefined (i.e.expected) distances between the peaks are stored in the sync sequencegap template in both the transmitter and the receiver. Thus for asynchronization sequence comprising N symbols, N-1 distance differencesare stored in both sync sequence gap templates. Note that the maximumlength synchronization sequence is 2704 phase units corresponding to 11USTs. Thus, the number of correlation windows to be applied to thereceived signal is 11. The term correlation window is defined as thesymbol time or UST. Note that alternatively the correlation window maybe chosen to be smaller or larger than a UST. If the minimum gap size iszero, for example, the correlation window should be smaller than a UST.

The acquisition mechanism will now be described in more detail. Forillustration purposes only, a sample received signal yielding a sequenceof correlation peaks is used. A diagram illustrating the output of thelinear correlator in response to a sample received signal correspondingto the synchronization sequence transmission signal of FIG. 4 is shownin FIG. 6. The ticks along the x-axis correspond to the 11 UST windows.The individual peaks are labeled PK#1 through PK#7 wherein PK#7 isgenerated later in time than PK#1.

The acquisition algorithm is operative to compare the position of eachreceived correlation peak against the expected position calculated inaccordance with the time delays of the synchronization sequence symbolgap template. The expected positions are calculated with reference tothe last received correlation peak, assuming it is in the correctposition. For example, the expected position of PK#6 is calculated bysubtracting gap #6 of the gap template from the position of receivedcorrelation PK #7. This distance is represented by reference numeral 90.Similarly, the expected position of PK#5 is calculated by subtractingthe sum of gaps #5 and #6 from the position of PK#7 (distance 92). Theexpected position of PK#4 is similarly calculated by subtracting the sumof gaps #4, #5 and #6 from the position of PK#7 (distance 94). In thismanner the expected distances of peaks PK#1 through PK#6 are calculated.Synchronization is declared if a sufficient number of matching peaks arefound. If synchronization is not found, the algorithm repeats assumingthe previous peak (i.e. PK#6) is correct, continuing until PK#4.

A flow diagram illustrating the acquisition method of the presentinvention in more detail is shown in FIGS. 7A and 7B. For each newcorrelation peak output of the linear correlator, the position and valueof the peak are stored in a table in memory (step 100). Each of thesetwo tables is sufficiently large to store 11 entries each correspondingto the possible 11 UST windows for the length of the synchronizationsequence. Initially, the algorithm does not start until at least 11windows have passed.

The algorithm begins with the last received correlation peak which isassumed to be in the correct position (step 102). In the examplepresented supra, PK#7 is assumed to be in the correct position. For eachof the other correlation peaks (i.e. peaks PK#6 through PK#1), theexpected position of each peak is calculated with reference to the lastreceived peak (i.e. PK#7) using the gap distance stored in the templatefor that symbol in the sequence (step 104). The expected positions ofthe earlier peaks in the sequence are calculated by subtracting the sumof the individual gaps stored in the template making up the distancebetween the earlier peak and the last received peak from the position ofthe last received peak. If the result is negative, a window length (i.e.1 UST or 256 phase units) is added to the position of the receivedcorrelation peak. This compensates for the case where a correlation peakwas not received in every window period. Since the positions of thecorrelation peaks are measured relative to the particular window inwhich they are received, a negative result indicates that at least onewindow UST passed wherein no correlation peak was received.

The expected position of the correlation peak is then compared with theposition of the actual received correlation peak (step 106). Theexpected (i.e. desired) position P_(EXP) of the correlation peak asderived from the synchronization sequence gap template is thensubtracted from the position of the received correlation peak P_(REC)(step 108). If the difference is within a predefined delta, a match isdeclared (step 110) and a counter num_matches is incremented. If thevalue of the correlation peak exceeds a peak value threshold (step 130),a num_high_peaks counter is incremented (step 132). If the difference isnot within the predefined threshold, a mismatch is declared and thenumber of mismatches is also tracked (step 112). Note that the delta maybe taken in either direction of the expected position, i.e. left orright, however, in the example embodiment presented herein, the delta istaken only to the left of the expected position. The value of the deltain the example presented herein equals 8 phase units. Other values forthe delta may also be used depending on the implementation.

The steps of calculating, comparing and determining if a match existsare repeated for each of the remaining correlation peaks (step 114). Inthe example case where num_peaks equals seven synchronization sequencesymbols, the process repeats six times for peaks PK#6 to PK#i. Once theprocessing for all the peaks is complete, synchronization is declared ifnum_matches is greater than num_matches_thresh (step 116). In thisexample, synchronization is declared if four or more matches out of sixare found, i.e. num_matches_thresh=3. Synchronization is also declaredin the borderline case when (1) num_matches=num_matches_thresh (e.g.,number of matches equals 3) and (2) half the matching peaks havecorrelation values above a threshold (e.g., the value of num_high_peaksequals 3).

If synchronization is declared, the point of synchronization iscalculated as described infra (step 134). The synchronization qualityfactor is also calculated (step 136). If the just calculatedsynchronization quality is better than the previously calculatedsynchronization quality (step 138), the previous synchronization pointis dropped and reception continues with the current synchronizationpoint (i.e. acquisition process continues) (step 140).

If synchronization is not found (step 116), the symbol corresponding tothe last received peak is assumed to have been received in error (step118) and the algorithm repeats and searches for matching peaks. In theexample presented, the symbol corresponding to PK#7 is assumed to havebeen received in error and PK#6 is considered the last received peak andis assumed to be in the correct position. The expected positions forPK#5 through PK#1 are then calculated with reference to PK #6 andcompared to the corresponding received peak positions. As in theprevious loop, the same criteria for synchronization apply here as well.If synchronization is not found, the next iteration assumes PK#7 andPK#6 were received in error, followed by the last iteration whereinPK#7, PK#6 and PK#5 are assumed to be received in error.

The iterations continue until peak #(num_peaks-num_matches). If afterthe last iteration, i.e.

PK#4 in this example, is considered the last received peak (step 120),synchronization has not been found, PK#7 is again considered the lastreceived peak and its position is moved one phase unit to the right(step 122). The algorithm is repeated with the new position for the lastreceived peak or peak #(num_peaks), i.e. PK#7 in this example. Ifsynchronization is not found, the position of the last received peak ismoved an additional phase unit to the right and the algorithm repeated.The shifting of the peak position continues up to delta phase units(step 124).

Note that moving one phase unit to the right refers to an advance on thetime axis (i.e. later in time) whereas moving one phase to the leftrefers to a retreat on the time axis (i.e. earlier in time).

Note that this shift of delta phases to the right combined with thepermitted delta for the difference between the expected position and thereceived position to the left of the expected position has the advantageof providing for a ±delta tolerance in the positions of the correlationpeaks while achieving a false alarm rate equivalent to ±delta/2.Alternatively, the false alarm rate may be reduced by permitting acertain width for the correlation peak rather than limiting the delta.

If synchronization is still not found after shifting the position of thelast received correlation peak, the algorithm is repeated using adifferent synchronization sequence (step 126). Thus, the expectedpositions of the received correlation peaks are calculated usingdifferent time delays associated with the synchronization sequence gaptemplate of the synchronization sequence being tested. The algorithm isrepeated until all synchronization sequences are tested (step 128). Ifsynchronization is still not found, the algorithm starts over again andwaits for a new correlation peak to be generated.

Note that the value of num_peaks may be varied depending on the level ofsensitivity desired. The lower the num_peaks, the higher the sensitivityof the acquisition to noise, etc. and vice versa.

Note also that in the above-described method, the correlation peaks areverified by looking for maximum linear correlation values.Alternatively, a search can be performed over the entire receivedsequence rather than on a symbol by symbol (or peak to peak) basis. Theentire sequence is examined against where the peaks are expected and asignal train is constructed comprising the deltas in the expectedlocations of the peaks. Thus, the search is performed across the entire11 USTs at once.

Note that the invention is not limited to the type of synchronizationquality measurement.

The synchronization quality measurement of counting the number of peaksexceeding a threshold is presented as an example. Alternatively, thecorrelation values at the expected position can be summed together andthe sum of all seven correlations compared to a threshold.

Tuning of the Synchronization Point

As described supra, the synchronization point is calculated oncesynchronization has been declared. One of the main purposes of theacquisition mechanism is to determine the synchronization point whichdefines the start of the actual packet. It is at this point, thatcorrelation of the received signal shifts from linear to cyclic and databegins to be decoded. Depending on the type of modulation used, thedistance between two consecutive symbols in the codebook may berelatively very small. For example, considering DCSK modulationtransmitting six bits per symbol, the distance between consecutivesymbols is only four phase units. Therefore, the point ofsynchronization must be determined with sufficient accuracy as a shiftof only 2 phase units in the synchronization point can lead tosynchronization error and loss of the entire packet. Choosing the wrongsynchronization point causes all symbols decoded to be shifted by oneposition in the codebook leading to incorrect decoding for all symbolsthat cannot be corrected by the error correction code.

Since the position of the last received correlation peak is permitted tovary a distance of delta phase units, the synchronization point can alsovary a distance of delta phase units. Thus, in accordance with theinvention, in the event of a match between the expected peak positionand the received peak position, information on the differences betweenthe expected peak position and the received peak position is used indetermining the synchronization point. A final tuning of thesynchronization point is performed using the average of the differencesfor matching peaks only as follows

$\begin{matrix}{{sync\_ pt}_{TUNED} = {{sync\_ pt} + \frac{\sum\left( {P_{E\;{XP}} - P_{REC}} \right)}{num\_ matches}}} & (1)\end{matrix}$wherein sync_pt is the untuned synchronization point. Thus, the averageof the shifts applied to the matching peaks is calculated and added tothe calculated synchronization point. Note that the untunedsynchronization point comprises the position of the last receivedcorrelation peak wherein synchronization was declared plus 1300 μs asdescribed supra.

Synchronization Quality Factor

In accordance with the invention, a synchronization quality factor iscalculated each time synchronization is declared. The quality factor isdefined as followssync_quality_factor=num_matches+num _high _peaks  (2)The quality factor is thus the sum of the number of matches and thenumber of correlation peaks in the correct position whose correlationvalue is greater than a threshold (e.g., 15 out of a 255 maximum). Thus,for a synchronization sequence of seven symbols, the quality factorranges between 0 and 12.

In accordance with the acquisition algorithm, the acquisition phasecontinues until receipt of the CRC8 at the end of the packet header. Ifthe CRC8 is correct, acquisition ends. If the CRC8 is in error,acquisition continues. If after synchronization is declared but still inacquisition, a new synchronization is declared with a quality factorhigher than that of the previous one, the packet previously beingreceived is dropped and the receiver immediately begins receiving thecurrent packet.

In order to continue performing acquisition after synchronization isdeclared until receipt of the CRC8, two sets of hardware are required.Alternatively, depending on the implementation, one set of hardware maybe used that is clocked at twice the nominal rate.

Criteria for Synchronization

The acquisition and synchronization mechanism of the present inventionis adapted such that the criteria for declaring synchronization ensuresthat the case does not occur whereby erroneous data in the packet can becorrected while synchronization was not achieved. In other words, thesynchronization algorithm is designed to be more reliable than datareception. In the example case where the error correction coding used isable to correct 2-3 erred symbols out of 7, the acquisition algorithm ispreferably more reliable.

Assuming the synchronization sequence comprises seven symbols, thecriterion for declaring synchronization is four correctly receivedsymbols. Thus, three erred symbols out of seven is permitted and thesituation where data can be corrected but synchronization was notachieved is prevented from occurring.

Further, in order to achieve a sufficiently low probability ofsynchronization from noise, the synchronization sequence is constructedusing time delays chosen so as to provide a high autocorrelationfunction for each sequence having low side peaks (i.e. no more than twoequal delays in one synchronization sequence).

Thus, in order to minimize the probability of synchronization due tonoise, the value of the correlation peak is used as an additionalcriterion. In particular, in the event of any three matches, if thecorrelation values of 3 out of 4 peaks exceed a predefined threshold,synchronization is declared, otherwise synchronization is deemed to becaused by noise.

In addition, in order to achieve a sufficiently low probability ofsynchronization from another synchronization sequence, thesynchronization sequence is constructed using time delays chosen so asto provide a low cross correlation function for each pair of sequences(i.e. the number of matches between the sum of consecutive delays or thedelay in one sequence and the sum of consecutive delays or the delay inother sequence should be minimal).

Thus, in order to minimize the probability of synchronization fromanother sequence, a minimum value of the correlation peak is used as anadditional criterion. In particular, in the event of any three matches(i.e. 3 erred symbols out of 7), the values of the correlation peaks ofthe four mismatches are checked. If the peak in error has a correlationvalue greater than a threshold (e.g., 30 out of 255 maximum) it isdeemed to have been caused by a peak from another synchronizationsequence and the declaration of synchronization is vetoed.

Station Incorporating the Mechanism of the Present Invention

The synchronization sequence generator and acquisition andsynchronization circuit of the present invention may be incorporated ina communications transceiver such as a station, network node, modem,etc. One example application is in a digital modem adapted forcommunications over the power line media. The modem utilizes a 100-400kHz band (for in the United States) or 95-125 kHz and 20-80 kHz band (inEurope). The modulation used is DCSK and the modem is capable ofunicast, broadcast and multicast transmissions using the spread spectrummodulated signal in the appropriate band. Each packet transmittedcomprises a synchronization sequence which permits the receiver tosynchronize on the spreading waveform (i.e. chirp, PN sequence, etc.)followed by the packet data modulated as circularly shifted dataspreading waveforms. As described above, the synchronization sequence isprocessed through a linear correlator while the data is decoded usingcyclic correlation.

A block diagram illustrating an example embodiment of a stationincorporating transmitter and receiver circuits adapted to perform theacquisition and synchronization sequence generation mechanisms of thepresent invention is shown in FIG. 8. The station, generally referenced150, represents a station that may operate stand alone or may beincorporated within a network device such as a switch, router, hub,broadband modem, cable modem, PLC based modem, etc. for performingcommunication functions (i.e. implementing OSI stack protocol functionsincluding MAC functionality). The station comprises an applicationprocessor 166 with associated static, dynamic, volatile and/ornon-volatile memory (not shown) in communication therewith. Theapplication processor is also in communication, via a host interface168, with a host device 170. The host may be adapted to communicate overone or more networks.

The station comprises media coupling circuitry 154 for interfacing thestation to the shared media 152. The transmit circuit 156 receives datafor transmission from the MAC and functions to encode the data intosymbols which are then modulated and transmitted over the media. Thetransmit circuit also comprises the synchronization sequence generator158 constructed in accordance with the present invention which functionsto generate the synchronization sequence transmitted at the start ofeach packet.

The transmit circuit 158 and receive circuit 160 communicate over themedia via the media coupling circuitry. The Rx circuit functions tocorrelate and decode the received signal and generate received outputdata therefrom. The receive circuit also comprises the correlation peakprocessor 161 and acquisition and synchronization circuit 162constructed in accordance with the present invention.

The media access controller (MAC) 164 functions, on one side, to providetransmit data to the transmit circuit and to input receive data from thereceive circuit. On the processor side, it interfaces to the applicationprocessor. The MAC is adapted to implement any suitable layer 2 (i.e.link layer) media access control technique as is well known in the art.

Note that the acquisition and synchronization sequence generationmechanism may be implemented in either hardware or software. Softwareimplementation may be adapted to reside on a computer readable medium,such as a magnetic disk, floppy disk, Flash memory card, EEROM basedmemory, bubble memory storage, RAM storage, ROM storage, etc. Thesoftware may also reside, in whole or in part, in the static or dynamicmain memories or in firmware within the processor of a computer system.The processor may comprise any suitable processing means includingmicrocontroller, microcomputer, microprocessor, digital signal processor(DSP), FPGA core, ASIC core, etc. In particular, the software comprisesa sequence of instructions which, when executed by the processor, causethe computer system to perform the acquisition and synchronizationmechanism described hereinabove.

In alternative embodiments, the present invention may be applicable toimplementations of the methods and apparatus described above inintegrated circuits, especially Application Specific Integrated Circuits(ASICs), Field Programmable Gate Arrays (FPGAs) or chip sets, wirelessmodem implementations, power line modem implementations, switchingsystem products and transmission system products. Note that acombination of software and hardware can also be implemented, the formerperforming the complex operations and the latter performing the timecritical operations.

For the purpose of this document, the terms switching systems productsshall be taken to mean private branch exchanges (PBXs), central officeswitching systems that interconnect subscribers, toll/tandem switchingcenters and broadband core switches located at the center of a serviceprovider's network that may be fed by broadband edge switches or accessmultiplexers and associated signaling and support system services. Theterm transmission systems products shall be taken to mean products usedby service providers to provide interconnection between theirsubscribers and their networks such as loop systems, and which providemultiplexing, aggregation and transport between a service provider'sswitching systems across the wide area, and associated signaling andsupport systems and services.

It is intended that the appended claims cover all such features andadvantages of the invention that fall within the spirit and scope of thepresent invention. As numerous modifications and changes will readilyoccur to those skilled in the art, it is intended that the invention notbe limited to the limited number of embodiments described herein.Accordingly, it will be appreciated that all suitable variations,modifications and equivalents may be resorted to, falling within thespirit and scope of the present invention.

1. A method of generating a start of packet synchronization sequence foruse in a transmitter, said method comprising the steps of: generating aplurality of N symbols to be transmitted in said synchronizationsequence, wherein N is a positive integer; generating N−1 predeterminedsignals, chosen to maximize autocorrelation properties of saidsynchronization sequence, wherein said N−1 predetermined signals conveypacket type information to a receiver; inserting one of said N−1predetermined signals after each of the first N−1 symbols in saidsynchronization sequence; encoding said synchronization sequence; andtransmitting said encoded synchronization sequence into a channel. 2.The method according to claim 1, wherein said predetermined signalscomprise time delays or transmitting gaps.
 3. The method according toclaim 1, wherein N equals seven.
 4. The method according to claim 1,further comprising generating a plurality of synchronization sequenceswherein each synchronization sequence corresponds to a unique set of N−1predetermined signals comprising time delays, each set of N−1 timedelays chosen so as to minimize cross correlation betweensynchronization sequences.
 5. The method according to claim 1, furthercomprising generating a plurality of synchronization sequences whereineach synchronization sequence corresponds to a different packet type. 6.The method according to claim 1, wherein said each symbol comprises azero shifted code shift keying modulated symbol.
 7. A method ofgenerating a start of packet synchronization sequence for use in a codeshift keying (CSK) based transmitter, said method comprising the stepsof: generating a plurality of symbols of known shift rotation to betransmitted in said synchronization sequence; inserting a respectivepredetermined time delay between each of said symbols; encoding saidsynchronization sequence; transmitting said encoded synchronizationsequence onto a channel; and wherein said respective predetermined timedelays inserted between said symbols are adapted to convey packet typeinformation to a receiver.
 8. The method according to claim 7, whereinsaid predetermined time delays are chosen to yield a synchronizationsequence having relatively high auto correlation properties.
 9. Atransmitter for use in a spread spectrum communications system,comprising: a synchronization sequence generator adapted to generate asynchronization sequence, said synchronization sequence representing aplurality of synchronization symbols with predetermined time delaysinserted therebetween; an encoder adapted to determine shift indices tobe applied to spreading waveforms, said shift indices determined basedon said synchronization sequence; a spreading waveform generator adaptedto generate spreading waveform signals in accordance with said shiftindices; and wherein delays between spreading waveform signals aredetermined by said predetermined time delays in said synchronizationsequence.
 10. The transmitter according to claim 9, further comprising asynchronization sequence gap memory adapted to store a plurality ofsynchronization sequences, each synchronization sequence comprising aset of symbols with predefined time delays between each of said symbols.11. The transmitter according to claim 9, implemented in an ApplicationSpecific Integrated Circuit (ASIC).
 12. The transmitter according toclaim 9, implemented in a Field Programmable Gate Array (FPGA).
 13. Amethod of generating a start of packet synchronization sequence, saidmethod comprising the steps of: generating a plurality of symbols to betransmitted in said synchronization sequence; inserting specific timedelays between said plurality of symbols in said synchronizationsequence, wherein said synchronization sequence selected to besubstantially orthogonal to other synchronization sequences;transmitting said synchronization sequence onto a channel; and whereinsaid specific time delays are adapted to convey information.
 14. Themethod according to claim 13, wherein said information comprises packettype information.
 15. The method according to claim 13, wherein saidsynchronization sequence comprises seven symbols having respective timedelays of therebetween.
 16. The method according to claim 13, whereinsaid synchronization sequence comprises seven symbols having respectivetime delays of therebetween.
 17. The method according to claim 13,wherein said synchronization sequence comprises seven symbols havingrespective time delays of therebetween.
 18. The method according toclaim 13, wherein said synchronization sequence comprises seven symbolshaving respective time delays of therebetween.
 19. The method accordingto claim 13, wherein said synchronization sequence comprises sevensymbols having respective time delays of therebetween.
 20. A method ofgenerating a start of packet synchronization sequence, said methodcomprising the steps of: generating a plurality of symbols to betransmitted in said synchronization sequence; inserting specific timedelays between said plurality of symbols in said synchronizationsequence, said time delays chosen to yield both a high autocorrelationfunction for a respective synchronization sequence and a lowcross-correlation function for each pair of synchronization sequences;and transmitting said synchronization sequence onto a channel.
 21. Themethod according to claim 20, wherein said specific time delays areadapted to convey information.
 22. The method according to claim 20,wherein said specific time delays are adapted to convey packet type. 23.The method according to claim 20, wherein said synchronization sequencecomprises seven symbols having respective time delays of therebetween.24. The method according to claim 20, wherein said synchronizationsequence comprises seven symbols having respective time delays oftherebetween.
 25. The method according to claim 20, wherein saidsynchronization sequence comprises seven symbols having respective timedelays of therebetween.
 26. The method according to claim 20, whereinsaid synchronization sequence comprises seven symbols having respectivetime delays of therebetween.
 27. The method according to claim 20,wherein said synchronization sequence comprises seven symbols havingrespective time delays of therebetween.